1. Field of The Invention
The subject invention relates to electronic ballasts for driving a high intensity discharge lamp, and more particularly, to controlling the output power in such electronic ballasts.
2. Description of The Related Art
It is typical to drive a high intensity discharge (HID) lamp with a low frequency square current waveform. An electronic ballast is used to generate the required drive waveform for the lamp and to provide power factor correction for the utility line. The present state of the art for electronic ballasts is the three-stage topology shown in FIG. 1. This topology consists of a boost converter for power factor correction, a buck converter for regulating lamp power/current, and a commutator for converting the regulated DC voltage into an AC square waveform. In this topology, two controllers are required. One controller is used in the boost converter to regulate the bus voltage, Vbus and to shape the line voltage waveform to follow the input voltage sinusoidal waveform for power factor correction. The other controller is used in the buck converter for regulating the lamp current and power. This topology is well known to those skilled in the art of power electronics. It is simple and straightforward to sense its control variables, because the line voltage is rectified and the sensed parameters can be referenced to the same potential throughout all phases of circuit operation. For example, a simple current sense resistor, R1 placed in series between ground and switch Q2, can be used to monitor the DC lamp current. However, the primary drawbacks for this topology are high parts count and low efficiency due to its multi-stage cascading nature. As shown in FIG. 1, the known three-stage converter ballast includes two controller switches, one for the boost converter and one for the buck converter, as well as four switches Q1-Q4 for controlling the commutation of the high intensity discharge lamp, for a total of six switches.